Loudspeaker structure

Abstract: A loudspeaker structure suitable for reducing a thickness of a loudspeaker, making it resistible to large vibration amplitudes, and providing a high performance. A repulsion magnetic circuit is formed by two magnets with the same poles being faced each other and a center plate interposed between the two magnets. The repulsion magnetic circuit is held in position by two frames. A diaphragm is mounted at the level flush with the center of the winding width of a voice coil. The outer periphery of an edge (suspension) bonded to the outer circumference of the diaphragm is held by flanges of the frames.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a structure of a loudspeaker having a repulsion magnetic circuit, the loudspeaker being thin and suitable for reproducing low frequency sounds and reproducing sounds at large vibration amplitudes. Related Background Art The structure of a conventional loudspeaker having a repulsion magnetic circuit is shown in Figs. 6 and 7. There is a strong desire to make a vehicle door-mounted loudspeaker thin and light in weight, because doors of a vehicle have become thin in order to increase the inner space of the vehicle and because the space for mounting the loudspeaker has become small in order to mount other devices, such as an automatic door locking mechanism and an automatic window opening/closing mechanism in the door. A small space in the door means a small space behind the diaphragm of a loudspeaker, resulting in a poor acoustic performance. As a result, sounds radiated from the loudspeaker, particularly low frequency sounds, are adversely affected. In order to intensify low frequency sounds, it is therefore necessary to make a loudspeaker as thin as possible and the effective space in a door as large as possible. Japanese Patent Laid-open publication No.1-98400 and the like teach that a repulsion magnetic circuit structure is effective in reducing the thickness of a loudspeaker, the structure having two magnets magnetized in the thickness direction and disposed with the poles of the same polarity being facing each other. As shown in Figs.6 and 7, according to this repulsion magnetic circuit structure, a flat diaphragm 150 is adhered to the upper end of a bobbin 141, and another diaphragm 152 is adhered to the lower end of the bobbin 141. The latter diaphragm 152 has an outer circumference area slanted and raised to the bottom surface of the flat diaphragm 150 where the diaphragms 150 and 152 are joined together. The repulsion magnetic field type loudspeaker shown in Fig.6 is structured such that a corrugation or edge extends radially from the outer circumference of the diaphragm 150. The diaphragm 150 and corrugation or edge are disposed above the bobbin 141 and a bobbin coil 140. A suspension, coil lead wires, input terminals 171, and the like are disposed under the bobbin 141. The repulsion magnetic field type loudspeaker shown in Fig.7 has a triple structure of magnets 131, 132, and 135 constituting a magnetic circuit and a two-stage structure of upper and lower voice coils 140 and 143 to which diaphragms 150 and 151 are adhered. This loudspeaker is therefore thick. In the case of the loudspeaker shown in Fig.6, it is necessary to keep a space necessary for accommodating the diaphragm 150 and corrugation above the voice coil 140 and bobbin 141 in the sound radiation direction, as well as an additional space for permitting the vibration of the diaphragm 150. Furthermore, it is necessary to keep a space under the voice coil 140 and bobbin 141 on the back side of the loudspeaker, this space being necessary for accommodating the diaphragm 152 and the coil lead wires and input terminals under the diaphragm 152. The coil lead wires are required not to contact the diaphragm 152, a frame 120, and the like. From these reasons, the loudspeaker shown in Fig.6 becomes thick. The above-described structure also limits a vibration amplitude of the diaphragm 150, and an available space left in the door is small as described above. Therefore, low frequency sounds in particular are difficult to be reproduced from a loudspeaker, and a power performance (maximum allowable input power characteristics) is very low, as compared to another loudspeaker having the same diameter and used in a larger space. In the case of the loudspeaker shown in Fig.7, the triple structure of the magnets 131, 132, and 135 and the two-stage structure of the voice coils 140 and 143 increase the thickness of the loudspeaker and reduce the space behind the loudspeaker. Furthermore, the outer circumferences of the diaphragms 150 and 151 are directly fixed to an outer frame 122 so that low frequency sounds are difficult to be reproduced more than the loudspeaker shown in Fig.6 and the power performance is extremely low. This loudspeaker is low in practical value as a door-mounted loudspeaker. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described conventional problems and provide a loudspeaker structure suitable for making a loudspeaker thin and resistible to large vibration amplitudes. According to the invention, there is provided a loudspeaker having two magnets magnetized in the thickness direction and disposed with the poles of the same polarity facing each other, which magnets generate a magnetic field; a center plate between the two magnets, a diaphragm means with a voice coil disposed in said magnetic field generated near the outer circumference of said center plate, and a suspension bonded to the outer circumference of said diaphragm means, characterized in that said diaphragm means and said suspension are disposed, in their rest position, between imaginary planes, which planes touch said coil at its extremities and are perpendicular to its axis. Preferably, a neck portion bonding said diaphragm means and said voice coil, the outer circumference of said diaphragm means, and the outer circumference of said suspension are position in the vicinity of a plane passing through the center of the winding width of said voice coil. The cross-section of said diaphragm means may have a width readily narrowing from near to a neck portion to said suspension. Furthermore, at least one of the front and back surfaces of said diaphragm means is corrugated concentrically with said voice coil. Also, at least one of the front and back surfaces of said diaphragm means may be provided with a rib of a drawing structure extending radially. Preferably, said diaphragm means is made of at least two diaphragms assembled together. To this end, a core member made of a foaming member or a core member having a honeycomb structure can be inserted between said at least two diaphragms. According to a preferred embodiment of the invention, said magnets are held by holding sections formed on two outer frames of the loudspeaker, at least one of said outer frames having a sound radiating area, and the outer circumference areas of said frames holding the outer circumference areas of said suspension. Said two frames may have the same shape. Further, said holding sections of said frames may be projections formed on the central area thereof and extending inwardly. Preferably, at least one of said sound radiating areas of said frames has punching holes. Input terminals of said voice coil are preferably mounted near the outer circumference of said suspension, the height of said terminals being lower than the thickness of said frames. Said magnets are preferably ring-shaped. According to the repulsion magnetic type loudspeaker of the present invention, the diaphragm and the suspension bonded to the outer circumference of the diaphragm are disposed at a level in the range from the top to the bottom of the voice coil, the magnets and the outer circumference of the suspension are held by the outer frames of the loudspeaker, the frames having the sound radiating areas. Accordingly, a space in which the diaphragm and the suspension vibrate, is limited to a space in which the voice coil vibrates, allowing the loudspeaker to be made thin. The neck portion bonding the diaphragm and the voice coil, the outer circumference of the diaphragm, and the outer circumference of the suspension are positioned near at the center of the winding width of the voice coil. Accordingly, the front and back areas of the loudspeaker unit become symmetrical and the frames of the same shape can be used. The cross section of the diaphragm has a width gradually narrowing near from a neck portion to the suspension. Accordingly, the cross section of the frames can be made thin toward the outer circumference thereof. At least one of the front and back surfaces of the diaphragm is corrugated concentrically with the voice coil, or is provided with a rib of a drawing structure extending radially from the inner circumference of the diaphragm. Accordingly, the rigidity of the diaphragm becomes high. The diaphragm is made of at least two diaphragms assembled together, and a core member made of a foaming member or a core member having a honeycomb structure is inserted between at least the two diaphragms. Accordingly, the rigidity of the diaphragm can be made higher. The two frames have the same shape so that the same metal mold can be used for manufacturing them. The holding sections of the frames are projections formed on the central area thereof and extending inwardly. Therefore, the magnetic circuit made of the magnets and the center plate can be reliably held in position by the projections, without using an additional support member for the magnets, thereby making the loudspeaker thin and light in weight. At least one of the sound radiating areas of the frames has punching holes, thereby realizing the loudspeaker having good high frequency characteristics. The input terminals of the voice coil are mounted at near the outer circumference of the suspension, and the height of the terminals is made so as not to exceed the maximum thickness of the frames. Accordingly, a mount space of the loudspeaker is less than the space corresponding to the maximum thickness of the frames. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a cross sectional view showing the structure of a loudspeaker according to an embodiment of the present invention. Figs.2A and 2B are a plan view showing the diaphragm unit with removed frames, and a side view showing the main part of the diaphragm unit. Fig.3 is a perspective view showing part of the voice coil and diaphragm of the embodiment. Figs.4A to 4C are front views showing the shapes of holes formed in a sound radiating area, the shapes including tiny punching holes shown in Fig.4A, circle holes shown in Fig.4B, and fan-shaped holes shown in Fig.4C. Fig.5 is a cross sectional view showing a general voice coil. Fig.6 is a cross sectional view showing a conventional loudspeaker structure. Fig.7 is a cross sectional view of another conventional loudspeaker structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the loudspeaker structure according to the present invention will be described with reference to Figs.1 to 5. Fig.1 is a cross sectional view showing the structure of a loudspeaker according to an embodiment of the present invention, Figs.2A and 2B are a plan view showing the diaphragm unit with removed frames and a side view showing input terminals, Fig.3 is a perspective view showing part of the voice coil and diaphragm of the embodiment, Figs.4A to 4C are front views showing the shapes of holes formed in a sound radiating area, the shapes including tiny punching holes shown in Fig.4A, circle holes shown in Fig.4B, and fan-shaped holes shown in Fig.4C, and Fig.5 is a cross sectional view showing a general voice coil. In Fig.1, reference numerals with affixes 1a and 1b represent magnets made of neodymium having the same ring shape. The two magnets 1a and 1b are magnetized in the thickness direction and disposed with the same poles being faced each other. A center plate 2 made of soft magnetic material such as iron is tightly held between the magnets 1a and 1b. A repulsion magnetic circuit is thus formed. The structure of the repulsion magnetic circuit will be detailed. The magnets 1a and 1b each have a size of an outer diameter of 29 mm, an inner diameter of 12 mm, and a thickness of 7 mm, and the center plate 2 has a size of an outer diameter of 30 mm, an inner diameter of 11 mm, and a thickness of 4 mm. The magnets 1a and 1b with the N poles facing each other and the center plate 2 held therebetween are fixed by adhesive agent. The magnetic circuit is fixed to a frame 7 by coating adhesive agent on the surface of a projection 7e (an outer diameter of 11.78 mm and a height of 2mm) formed at the central area of the frame 7 and by fitting the hole of the magnet 1a around the projection 7e. A bobbin-less voice coil 3 uses a slit wire of aluminum having an edgewise (rectangle section) structure. The voice coil 3 has a coil winding unit 3b which has no bobbin and has an inner diameter of 30.5 mm, a winding width of about 5 mm, and a d.c. resistance of 3.4 Ω. An insulating tape 3d is attached to the outer side wall of the voice coil 3, and two copper foils 3c are attached to the surface of the insulating tape 3d disposed side by side and spaced apart from each other. The winding start and end of the coil are soldered to the copper foils 3c. A diaphragm 4 is constituted by front and back diaphragms 4a and 4b. A core member 4c is interposed between the diaphragms 4a and 4b. The core material 4c is made of balsa worked in a predetermined shape. The balsa has voids and is light in weight. Pulp sheets compression-molded into a shape matching the outer shape of the core member 4c are attached on the front and back surfaces of the core member 4c. In this manner, the diaphragm 4 is formed which has a high rigidity and can suppress local resonances. As shown in Fig.1, the cross section of the diaphragm 4 has a shape symmetrical to a lateral center line thereof, having a thickness of about 4 mm at the inner circumference area and gradually reducing its thickness toward the outer circumference. The overall shape of the diaphragm 4 is a disk having an outer diameter of about 110 mm. The diaphragm 4 is thinner than the width of the voice coil 3, and the inner circumference area (neck area) of the diaphragm 4 is adhered to the central area of the outer wall of the voice coil 3. An edge 5 providing a suspension function is joined at its inner circumference area to the outer circumference area of the diaphragm 4, the outer circumference area of the edge being fixed to a flange 7f of the frame 7 via an insulating ring 9 made of Bakelite and having a thickness of 1.5 mm. Lead wires 6 connect the copper foils 3c attached on the outer wall of the voice coil 3 to input terminals 11. The lead wire 6 is made of a woven cloth (edge member) or the like of an insulating material and a conductive, flexible, and flat woven tinsel wire sewed on the woven cloth. The lead wires 6 are molded by hot pressing into a shape matching the shape of the diaphragm 4 and edge 5, and attached to the surface of the diaphragm 4 and edge 5 extending in generally parallel from the copper foils 3c to the input terminals 11. As shown in Fig.3, the woven cloth is trimmed so as to have a width slightly wider than that of the flat woven tinsel wire. Another pair of lead wires 6 are attached also on the side having no input terminal as shown in Fig.2A, to thereby provide a dynamic balance of the loudspeaker. The two lead wires 6 are soldered to the copper foils 3c corresponding the winding start and end of the coil winding unit 3b at the inner circumference area of the diaphragm 4, and also to the input terminals 11 of two lug plates shown in Fig.2B. In this manner, an electrical connection between the winding start and end and the input terminals 11 can be established. In assembling the diaphragm 4 and the voice coil 3, the voice coil 3 is first loaded in an assembly jig (not shown), and then the inner circumference of the diaphragm 4 is fitted around the outer wall of the voice coil 3. In this state, the copper foils 3c attached to the outer wall of the voice coil 3 and soldered to the winding start and end thereof are soldered to the flat woven tinsel lead wires 6 extended to the inner circumference of the diaphragm 4. In addition, the C-plane areas formed at the inner circumference area of the diaphragm 4 on both the front and back surfaces are coated with adhesive agent to bond the voice coil 3 and diaphragm 4. This vibration system is then mounted to the frame 7 to which the magnetic circuit has been adhered. Another frame 8 having the same shape as the frame 7 is mounted to thus complete the loudspeaker. The edge 5 fixed to the ring 9 of Bakelite at the flange 7f of the frame 7 is substantially flush with the center of the width of the voice coil 3. In the loudspeaker assembled as above, the center of gravity of the vibrating system is located on the plane passing through the center of the winding width of the voice coil. The position of this center of gravity functions as the driving point of the loudspeaker. Therefore, only the outer periphery suspension such as the edge 5 can support the vibrating system, without using a conventional inner periphery suspension such as a damper, and a rolling at a large vibration amplitude is small. The frames 7 and 8 have the same shape so that they can be manufactured economically by using the same metal mold. The acoustic filter characteristics of the loudspeaker can be changed by forming holes having different shapes in sound radiating areas 7a and 8a of the frames 7 and 8. Specifically, as shown in Figs.4A to 4C, punching holes 7b and 8b of a rectangle shape or a circle shape as illustratively shown in Fig.4A, circle holes 7c and 8c as shown in Fig.4B, or fan-shaped holes as shown in Fig.4C are used, or the shapes of holes may be changed between the two frames 7 and 8. The high frequency band characteristics of the loudspeaker can be controlled as desired by using punching holes 7b and 8b. A sample of a 16 cm loudspeaker having the structure of this invention showed a resonance frequency fo of 60 Hz enabling to move the lower frequency limit to a sufficiently low value, provided a stroke (amplitude) of the diaphragm of about 12.5 mm, had a total thickness of 20 mm or less including a frame thickness, and ensured a rated maximum allowable input power of 50 to 70 W without using an inner periphery suspension (damper). In this embodiment, although the voice coil 3 having no bobbin has been described, the invention is not limited only to it, but a bobbin 3a such as shown in Fig.5 may be used. In this embodiment, the cross section of the diaphragm 4 is of a wedge shape gradually reducing the thickness from the inner circumference area to the edge 5, and the inner circumference area and edge 5 are fixed to the center of the winding width of the voice coil 3. Instead, the inner circumference area and edge 5 may be fixed to any position in the range of the winding width of the voice coil 3. In this embodiment, the diaphragm 4 has a sandwich structure disposing the core member 4c inside of the diaphragm 4. The structure of the diaphragm 4 may be changed in accordance with an application object, or the structure of the core member 4c 4c may be changed. For example, the core member 4c may have a honeycomb structure, the core member 4c may be omitted, the front and back diaphragms 4a and 4b may have different shapes, or the diaphragm 4 may be formed by a single diaphragm. In this embodiment, although the flat woven tinsel lead wires 6 are extended in parallel, each lead wire 6 may be extended symmetrically relative to the central axis of the voice coil 3 by mounting two lug terminals at the corresponding positions. A plurality of lead wires 6 may be used if necessary. According to the present invention, the projections 7e and 8e for holding the magnets 1a and 1b are extended to the inside of the frames 7 and 8. Instead, the projections may be extended to the outside of the frames to hold the magnets at their outer walls. The following advantageous effects can be obtained by the loudspeaker structure of this invention. (1) The repulsion magnetic circuit is held by the frames 7 and 8 at the central areas thereof, the edge 5 at the outer periphery of the diaphragm is fixed to the flange 7f, and the lead wires 6 of the voice coil 3 are attached to the diaphragm 4 and edge 5. Accordingly, the diaphragm 4 and edge 5 can be positioned in the range of the winding width of the voice coil 3, and the assembly of the lead wires 6 can be achieved in this configuration. The components such as the magnetic circuit and input terminals 11 can be accommodated in a space within the vibration amplitude range of the voice coil 3. The structure of a loudspeaker which is theoretically thinnest can be provided. (2) The thin loudspeaker of this structure is resistible to large vibration amplitudes and has a very high maximum allowable input power performance. (3) Since the edge 5 is positioned at the center of the winding width of the coil winding unit 3b, a rolling of the vibration system is not likely to occur. Accordingly, the loudspeaker structure excellent in reproducing low frequency sounds can be provided even for a thin loudspeaker. (4) A desired sound quality can be obtained by changing holes formed in the sound radiating areas of the frames 7 and 8 having the same shape. The high frequency characteristics of the loudspeaker can be set as desired by forming punching holes 7b and 8b in the sound radiating areas. (5) The repulsion magnetic circuit of the loudspeaker structure has fewer components, and lead wires are soldered for the electrical connection. Accordingly, assembly of the loudspeaker is simple and the costs of both components and assembly works can be reduced. (6) In this loudspeaker structure, lead wires are attached to the diaphragm so as to reliably prevent the breakage of the lead wires and the generation of abnormal sounds. (7) The frames 7 and 8 are formed with the projections 7e and 8e for holding and position-aligning the magnetic circuit. Accordingly, the loudspeaker can be assembled easily. Since the magnetic circuit is squeezed by the frames 7, a sufficient mechanical strength can be easily obtained even if thin frames are used. (8) The lead wire 6 to the voice coil 3 is made of the insulating woven cloth (edge material) and the flat woven tinsel wire sewed on the woven cloth which is attached to the surface of the diaphragm 4. Accordingly, even if the material of the diaphragm is conductive, such as carbon fiber, aluminum alloy foil, and the like, there is no fear of a short-circuit.

Claims: A loudspeaker having two magnets (1a, 1b) magnetized in the thickness direction and disposed with the poles of the same polarity facing each other, which magnets generate a magnetic field; a center plate (2) between the two magnets, a diaphragm means (4) with a voice coil (3) disposed in said magnetic field generated near the outer circumference of said center plate (2), and a suspension (5) bonded to the outer circumference of said diaphragm means (4), characterized in that said diaphragm means (4) and said suspension (5) are disposed, in their rest position, between imaginary planes, which planes touch said coil at its extremities and are perpendicular to its axis. The loudspeaker according to claim 1, wherein a neck portion bonding said diaphragm means (4) and said voice coil (3), the outer circumference of said diaphragm means (4), and the outer circumference of said suspension (5) are positioned in the vicinity of a plane passing through the center of the winding width of said voice coil (3). The loudspeaker according to claim 1 or 2, wherein the cross section of said diaphragm means (4) has a width gradually narrowing from near to a neck portion to said suspension (5). The loudspeaker according to any preceding claim, wherein at least one of the front and back surfaces of said diaphragm means (4) is corrugated concentrically with said voice coil (3). The loudspeaker according to any preceding claim, wherein at least one of the front and back surfaces of said diaphragm means (4) is provided with a rib of a drawing structure extending radially. The loudspeaker according to any preceding claim, wherein said diaphragm means (4) is made of at least two diaphragms (4a, 4b) assembled together. The loudspeaker according to claim 6, wherein a core member (4c) made of a foaming member or a core member having a honeycomb structure is inserted between said at least two diaphragms (4a, 4b). The loudspeaker of any preceding claim, wherein said magnets (1a, 1b) are held by holding sections (7e, 8e) formed on two outer frames (7, 8) of the loudspeaker, at least one of said outer frames (7, 8) having a sound radiating area (7a, 8a), and the outer circumference areas of said frames (7, 8) holding the outer circumference areas of said suspension (5). The loudspeaker according to claim 8, wherein said two frames (7, 8) have the same shape. The loudspeaker according to claim 8 or 9, wherein said holding sections (7e, 8e) of said frames (7, 8) are projections formed on the central area thereof and extending inwardly. The loudspeaker according to claim 8, 9 or 10, wherein at least one of said sound radiating areas (7a, 8a) of said frames (7, 8) has punching holes (7c, 8c). The loudspeaker according to claim 8, 9, 10 or 11, wherein input terminals (11) of said voice coil (3) are mounted near the outer circumference of said suspension (5), the height of said terminals (11) being lower than the thickness of said frames (7, 8). The loudspeaker according to any preceding claims, wherein said magnets (1a, 1b) are ring-shaped.

Inventors: OHTA SHUHEI, SAKAMOTO YOSHI, OHTA, SHUHEI, SAKAMOTO, YOSHI
Applicants: KENWOOD CORP, KABUSHIKI KAISHA KENWOOD
Application year: 1994
Main classification: H04R7
IPC codes: H04R9, H04R7